Field of the Invention
The present invention relates to a method of producing p-n junction semiconductor solar cells. More particularly, the invention relates to a method of producing solar cells with the principal objective of sharply reducing production costs by depositing polycrystalline silicon on a relatively cheap substrate such as metallurgical-grade silicon, graphite or steel.
Description of the Prior Art
The problem of uncovering new, abundant, cheap and non-polluting sources of energy is a problem of vital national importance. Of all energy sources, solar energy is one of the more attractive sources because of its abundant supply and because it is completely non-polluting. An indication of the abundance of solar energy is evident by the fact that the solar power on the surface of the earth is approximately one hundred thousand times greater than the current power consumption from all energy sources.
Presently, solar energy is utilized by converting solar energy to thermal energy and by converting solar energy to electricity which is known as the photovoltaic system. Both methods of utilizing solar energy are expected to aid in meeting the ever-increasing demand for clean solar energy. Currently, the silicon solar cell is the most well-known device in the photovoltaic system. Further, technology has advanced to the point where silicon solar cell panels which are capable of producing several kilowatts of power have been used reliably in all types of space craft for many years.
Currently, silicon solar cells are manufactured by preparing polycrystalline silicon by reducing trichlorosilane with hydrogen, growing single crystals of silicon of controlled purity from the polycrystalline material, preparing silicon wafers by cutting the single crystal ingot to a thickness of at least 0.25 mm followed by polishing and etching, diffusing a dopant into the silicon wafers to form a shallow p-n junction, applying ohmic contacts to the rear surface and grid contact to the diffused surface, applying antireflecting and protective coatings to the diffused surface and finally mounting the cell into position. This rather intricate procedure results in the current high costs of manufacturing silicon solar cells. Although the costs of production for single crystalline solar cells has recently been reduced from about $100/peak watt to about $20/peak watt, further reduction in cost of about one order of magnitude is necessary if widespread utility of solar cells is to be realized in large-scale terrestrial applications.
One prior art process of manufacturing semiconductor solar cells as shown by Tarneja, et al., U.S. Pat. No. 3,460,240, involves epitaxially depositing silicon on a quartz substrate to form an N-type layer over which is epitaxially deposited two P-type silicon layers. However, this process has the disadvantage that the overall process requires the rather detailed and expensive sequence of steps necessary to deposit epitaxial silicon so that no significant decrease in cost of manufacture is observed.
The Jones reference, U.S. Pat. No. 3,078,328, shows a method of manufacturing solar cells in which a layer of silicon is grown onto a graphite surface from a silicon melt and doped to form an N-type layer. In the growth step, silicon and carbon at the interface of the silicon and graphite layers mix to form an intermediate layer of silicon carbide. The device is completed by formation of a top p-type layer of silicon by diffusion. The reference again is disadvantaged by the complicated fabrication procedure. Thus, the cost of manufacture is unattractive from a commercial viewpoint.
Small-area polycrystalline silicon solar cells have also been fabricated by the deposition of silicon from a vapor state reactant. A polycrystalline silicon layer of a thickness of 25 - 50 .mu.m was deposited on silicon substrates at 900.degree.C by the reduction of trichlorosilane with hydrogen. In this method, silicon substrates were used for convenience in order to eliminate the cracking of silicon which has been deposited on other substrates. By this procedure, 1 cm.sup.2 solar cells were fabricated by the successive diffusion of gallium and phosphorous to form a p-n junction about 2.5 .mu.m below the surface of the device. The device had a maximum open-circuit voltage of about 0.3 V, and the greatest efficiency was about 0.9%.
A need continues to exist, therefore, for a method by which silicon semiconductor solar cells can be easily and cheaply produced.